The present invention is generally in the field of solar energy collecting systems of the so called focused collector type. More particularly, the invention is concerned with a radiation heat-shield for such collectors.
Rapid exploitation of natural energy resources together with the search for environmental friendly energy resources and the need to provide energy to remote communities or plants raise an increase interest in solar energy systems and in improvements thereto. However, owing to considerations of economics and efficiency, commercial solar energy is still limited in use. Nevertheless, continuous research increases efficiency of such solar systems which gradually become more cost effective and render them more favorable.
Various types of solar collecting systems are known for converting solar energy into other forms of useful energy, by collecting the solar energy and transforming it into heat energy. The heat energy may then be consumed in various forms.
One particular type of solar systems is the so-called xe2x80x9cfocused collector typexe2x80x9d wherein a beat collector element (HCE) is received at the focus of a reflecting member which in order to improve the angular radiation efficiency of the solar system, tracking means are provided for tracking the sun as it progresses in the sky. The HCE comprises a spatter coated tube with a liquid flowing therethrough, and in order to still improve the efficiency of the solar system, the coated tube is received within a coaxial translucent protective tube (typically made of durable glass) with a vacuum formed therein. Such systems are often referred to as ultimate vacuum collectors (UVAC).
The glass protective tube, protects the coated tube which is made of metal, coated with a layer of material having a high radiation absorbing coefficient and minimizes heat loss from the coated tube. A solar heat collector system is typically tens of meters long and occupies a large field. For practical reasons the solar system is constructed of aligned coated tube elements connected to one another so as to form a continuous line, whilst segments of the protective glass tubes extending over a major portion the coated tube elements, though they are not continuous.
The coated tubes and the protective tubes have different heat expansion coefficient, and accordingly the protective tubes are not continuous, since temperature changes would result in non-uniform expansion of the coated tubes and their respective protective tubes, resulting in severe damage to the system. Thus, it becomes necessary to sealingly couple the protective tubes over the coated tube utilizing a suitable device which provides some degree of freedom, in particular in an axial direction, so as to allow for different expansion of the glass tube over the metal coated tube, whilst retaining the vacuum within the protective tube.
Such an element is a bellows-type connector element having one axial end thereof sealingly secured over the coated tube and its opposed end co-axially connected to the protective tube by a so-called glass to metal connection, wherein the thickness of the respective end of the bellows is significantly reduced and has a diameter corresponding with that of the glass tube. By melting the end of the glass tube, the reduced thickness metal portion may be introduced into an edge of the molten glass tube whereby, its edges becomes overlapped by glass and in fact becomes sealingly welded thereto.
However, the zone of the glass to metal connection remains vulnerable and temperature changes may cause different expansion of the metal versus the glass components, resulting in loss of vacuum or even in rupturing of the envelope glass protective tube.
The problem of different expansion coefficient of glass and metal components may occurs in particular by concentrated or reflected radiation, which instead of being reflected towards the coated tube of the heat collecting element, are reflected to the glass to metal connection zone. A particular problem occurs when the sun is at low angles, where radiation is reflected towards one of the glass to metal connection zones of the HCE. For example, since it is most efficient to place the solar system such that a longitudinal axis of the HCE is positioned parallel with the meridian, i.e. extending in a north-south orientation, the problem of rays striking against the glass to metal connection zone occurs in the northern hemisphere mainly at the northern ends of each protective tube.
It is an object of the p invention to provide a radiation shield to protect the glass to metal connection zone from sunrays, direct or reflected, so as to prevent non-uniform expansion of glass and metal components at the connection zone.
According to the present invention there is provided a radiation shield for a solar system, to be attached to a connecting element sealingly retaining the protective tube over the heat collecting element, whereby the radiation shield is attached thereto in a manner and location which adequately shields the glass to metal connection zone in spite of axial displacement of the connecting element, also at extreme low radiation angles of the sun, of both direct and reflected sunrays, whilst interferes as little as possible with the effective collecting area of the HCE, so as not to deteriorate the overall efficiency of the solar system.
According to the present invention there is provided a solar system comprising a reflector with a beat collecting element (HCE) received at the focus of the reflector, said HCE comprising coated tube having a longitudinal axis extending between a first end and a second end; the coated tube being received within a coaxial evacuated enveloping glass tube; said enveloping glass tube being secured at respective ends thereof to the coated tube by a coaxially deformable connector element having a distant end thereof sealingly secured to the HCE and a proximal end thereof sealingly attached to a respective end of the enveloping glass tube by a glass to metal connection (GMC), wherein a radiation shield assembly extends over the connector element and over the GMC; said radiation shield assembly is secured to the connector element by support legs engaging the connector element adjacent the proximal end.
According to one embodiment of the invention, the shield assembly comprises an external unitary shield member extending over the deformation zone and the GMC zone. According to a different embodiment, the radiation shield assembly comprises a first eternal shield member extending from the proximal convolution towards the distal end of the connector element, and a second external shield member extending from the proximal convolution towards a corresponding end of the glass tube. Accordingly, the fit shield member shields the connector element and the second shield member shields the GMC zone.
One particular feature of the invention is to provide a radiation shield assembly comprising a first shield member extending over the connector element (deformation zone), and a second shield member extending over the transition zone and the GMC zone.
According to another aspect, the invention provides also an internal shield member extending between the HCE and the enveloping glass tube adjacent the GMC zone; said internal shield member intersecting the longitudinal axis. The internal shield member is in particular useful for shielding the glass to metal connection zone from rays reflected when the sun is at low angles.
The external shield members are formed, according to an embodiment of the invention, with a plurality of support legs, radially projecting inwardly. Said support legs, according to some embodiments of the invention, are integral with the external shield members. However, for the internal shield member, said support legs may be non integral with the shield ring and may be attached thereto in a manner imparting them radial biasing effect.
For best results, at least external surfaces of the shield assembly members are reflective, e.g. by means of a solar reflective coating agent, by suitable finishing (polishing) of high reflective solar metals, etc.
The invention is also concerned, by another of its aspects, with the construction of shield members and their manufacturing.